The macronuclear genetic system of the Protozoan, Oxytricha, is unique among eukaryotes in that the DNA is segmented into small, approximately gene-sized pieces, rather than being in large chromosomal molecules. These small pieces are very active in transcription. This research project is a study of the molecular organization of these DNA pieces, and the relationship of molecular structure to the control of gene expression. The ribosomal RNA genes are found to be contained in DNA pieces with a molecular weight of about 5 x 10 to the 6th power, scarcely larger than the sequence coding for the rRNAs themselves. Similarly, the 5s RNA genes are within DNA pieces which are, at most, a few hundred base pairs longer than the 5s RNA gene. Through this kind of analysis it is possible to estimate the size of those DNA sequences associated with, but not part of, structural genes. This work is continuing, both for the rRNA and 5s RNA genes, and for other structural genes of interest. Combined with our studies on macronuclear gene activation and inactivation, these results should give valuable insight into the DNA sequences involved with regulation of gene expression.